1. Field of the Invention
The invention pertains generally to optoelectronic integrated circuits, i.e., circuits in which optical devices, such as photodetectors, are integrated with one or more electronic devices, such as transistors.
2. Art Background
Optical communication systems, e.g., optical fiber communication systems, include photodetectors which serve to convert optical signals into electrical signals, which are processed using electronic integrated circuits (ICs). The photodetectors employed in such systems have conventionally been discrete devices, formed in substrates different from those employed in forming the electronic ICs, with the discrete photodetector devices being electrically connected to the electronic ICs via electrical wires. In this regard, to reduce costs and enhance performance, there has been a desire to integrate the photodetectors with the electronic ICs, i.e., to form both on the same substrate, and thus form monolithic, optoelectronic ICs.
In optical communication systems operating at wavelengths of 0.8 micrometers (.mu.m), the photodetectors have typically been fabricated in substrates of GaAs, a III-V compound semiconductor material which absorbs light of this wavelength. Significantly, a GaAs photodetector has been fabricated in a metal-semiconductor-metal (MSM) device configuration, which permitted ready integration with metal-semiconductor field effect transistors (MESFETs). (See D. L. Rogers, "Monolithic Integration of a 3-GHz Detector/Preamplifier Using a Refractory-Gate, Ion-Implanted MESFET Process", IEEE Electron Device Letters, Vol. EDL-7, No. 11, November 1986, pp. 600-602.) That is, the metal electrodes of the photodetector (used to collect the electrical charge carriers produced by the absorption of light) were formed on the same surface of a layer of GaAs, with the metal electrodes being spaced apart from one another to permit absorption of light by an intervening region of GaAs. In addition, MESFETs were formed on the same layer surface, adjacent, and electrically connected, to the MSM photodetector.
Among other characteristics, the MSM photodetector of the optoelectronic IC, described above, exhibited a relatively high signal-to-noise (S/N) ratio. That is, the metal electrodes formed Schottky contacts (rectifying metal-semiconductor contacts) to the GaAs, and the corresponding potential barriers to the emission of electrons and holes from the metal electrodes into the GaAs were relatively high. As a consequence, the dark current the current which flows through the photodetector in the absence of light) was relatively small, resulting in a relatively high S/N ratio.
To reduce the effects of dispersion and/or optical loss, long distance optical fiber communication systems employing silica optical fibers are now being operated at wavelengths of 1.3 .mu.m and/or 1.55 .mu.m. Significantly, the corresponding photodetectors are fabricated in ternary and quaternary III-V compound semiconductors, i.e., ternary alloys in the In-Ga-As system and quaternary alloys in the In-Ga-As-P system. Unfortunately, when metal electrodes are formed on these ternary and quaternary semiconductors, the resulting Schottky contacts exhibit relatively small potential barriers to electrons in the metal electrodes. As a result, the corresponding dark currents in these MSM photodetectors are relatively high and the S/N ratios are relatively low.
Attempts have been made to reduce dark currents in MSM photodetectors formed in the ternary and quaternary semiconductors. In one such attempt, a layer of GaAs was formed on a photoabsorbing layer of In.sub.0.53 Ga.sub.0.47 As, and the metal electrodes were formed on the GaAs. (See H. Schumacher et al, "An Investigation of the Optoelectronic Response of GaAs/InGaAs MSM Photodetectors," IEEE Electron Device Letters, Vol. 9, No. 11, November 1988, pp. 607-609). While this procedure reduced dark currents, the procedure is considered less desirable because the substantial lattice mismatch between GaAs and In.sub.0.53 Ga.sub.0.47 As introduces undesirable strain and misfit dislocations into the former.
In another attempt to reduce dark currents, a layer of In.sub.0.52 Al.sub.0.48 As was formed over a photoabsorbing layer of In.sub.0.53 Ga.sub.0.47 As, the former serving to enhance the potential barrier to electrons at the metal-semiconductor interface. To reduce carrier trapping caused by the relatively large band discontinuity at the In.sub.0.52 Al.sub.0.48 As/In.sub.0.53 Ga.sub.0.47 As interface, a graded superlattice, consisting of ultrathin layers of InAlAs and InGaAs, was incorporated between the In.sub.0.52 Al.sub.0.48 As and In.sub.0.53 Ga.sub.0.47 As layers. (See O. Wada et al, "Very high speed GaInAs metal-semiconductor-metal photodiode incorporating an AlInAs/GaInAs graded superlattice," Appl. Phys. Lett., Vol. 54, No. 1,2 January 1989, pp. 16-17.) While it has been asserted that this photodetector exhibits a relatively low dark current, its manufacture is relatively complicated. That is, the formation of the In.sub.0.52 Al.sub.0.48 As, a ternary, is inherently more difficult and more complicated that that of, for example, a binary. In addition, the fabrication of the graded superlattice necessarily requires the use of a relatively sophisticated technique, such as molecular beam epitaxy (MBE).
Thus, those engaged in developing optoelectronic ICs have sought, and continue to seek, photodetectors having configurations which result in relatively low dark currents, permit ready integration with electronic ICs, and are relatively easy to fabricate.